102 FRESHWATER ANIMALS 



(Brooks, 1956). It is difficult to believe that a character mani- 

 fest in such sporadic fashion could be of such adaptive sig- 

 nificance that the ability to produce this character would be 

 carried latent in the germplasm of a species with an extensive 

 distribution over two continents. 



We fortunately know something of the physical properties of 

 the water in two of the lakes in which populations with retro- 

 curved helmets occurred. We are thus in a position to compare 

 the environmental characteristics at the time of helmet develop- 

 ment with the more usual conditions under which helmets are 

 not produced. This comparison is made in diagrammatic fashion 

 in Fig. 7. Under a specimen representing the helmet character- 

 istic of each population is depicted half of a vertical section 

 through the lake in which the population lives, and a graph 

 showing the temperature at different depths. The section at the 

 left is characteristic of the majority of lakes of the temperate 

 zone in midsummer. The graph indicates that an upper stratum 

 of water is warm, about 20° C; below this there is a region 

 where the temperature decreases rapidly with increasing depth. 

 Most of the bottom waters are slightly above the temperature of 

 maximum density (4° C). The vertical hatching on the lower 

 part of the section indicates that D. longiremis usually lives in the 

 cool bottom waters. D. retrocurva, on the other hand, would live 

 in the warm, turbulent upper layers. Such populations, living in 

 cool waters in summer as well as in winter, show little seasonal 

 variation, and at most develop the small crest indicated. 



The center section represents the situation in Muskellunge 

 Lake, Wisconsin, on August 26, 1931. This was reported, but 

 differently interpreted, by Woltereck (1932). Here Daphnia 

 longiremis with tall, slightly retrocurved helmets occurred, and 

 as we might expect, the population was living in warmer (and 

 more turbulent) water than the species usually does. The bot- 

 tom waters were devoid of oxygen, and the population could 

 maintain itself only in the middle strata of water. The diagram 

 at the right represents the physical conditions in Lake Waskesiu, 

 Saskatchewan, during midsummer of 1932 and the helmeted 

 longiremis that developed therein. The thermal conditions in 



